The present invention relates to a method for transmitting information in a radiocommunication system, the information being transmitted in compressed form between a transmitter and a receiver, and to a corresponding radiocommunication system, particularly a corresponding mobile radiocommunication system.
It is known that information is transmitted embedded in a frame structure in mobile radiocommunication systems. The frame structure includes a number of frames transmitted sequentially, each frame exhibiting a particular number of time slots. It has been proposed with regard to known mobile radiocommunication systems to operate the transmitter of the mobile radiocommunication system in a compression mode (slotted mode) where the information to be transmitted is transmitted in compressed form, within the frames, to a receiver, thereby freeing in the corresponding frame a section of information bits (i.e., the idle slot) which can be subsequently used for interfrequency measurements (i.e., measurements at other frequencies), for example, for preparing a handover between different mobile radiocommunication systems. To utilize the compression, the information must be transmitted in a shortened time interval. In this regard, various approaches are known and will be explained in greater.
FIG. 4 shows a simplified block diagram of the components involved in the coding/decoding of voice signals in a mobile radiocommunication system. The information to be transmitted via a radio-frequency transmission link 12 is initially converted into a bit sequence via a digital source encoder 9. If the information to be transmitted is voice information, a voice encoder 9 is correspondingly used. At the receiver end, a voice decoder 15 is correspondingly provided. The source-encoded or voice-encoded data are then coded with the aid of a channel encoder 10, additional redundant bits being added to the actual information bits, thereby allowing transmission errors to be detected and subsequently corrected. A corresponding channel decoder 14 is provided at the receiver end. Before the channel-encoded information is transmitted to the receiver, it can be supplied to an interleaver 11 which rearranges the data elements to be transmitted in time in accordance with a particular arrangement and at the same time spreads them in time. As a result, the errors, which, as a rule, occur in bunches, are distributed in order to obtain what is known as a memoryless transmission channel with a quasi-random error distribution, since this is presupposed by most of the channel codes. Finally, the radio-frequency transmission link or air interface 12 includes a transmit unit in the transmitter and a receiving unit in the receiver, the transmit unit modulating the data supplied to it and to be transmitted onto a carrier signal and transmitting them in accordance with a particular multiple access method to the receiving unit which down-converts the received signal again to the baseband and supplies it to a deinterleaver 13.
Thus, a first approach to compressing the information to be transmitted provides for using a higher coding rate during the channel coding in the transmitter. Since the number of information bits per frame remains constant when using a frame structure having a number of frames transmitted sequentially, what is known as an idle slot or idle section, (i.e., a section not occupied with information) that can be used for, among other things, interfrequency measurements in order to, for example, prepare a handover between different mobile radiocommunication systems, is generated with the higher coding rate in the compressed frame. This operating mode is also known as xe2x80x9cslotted modexe2x80x9d and is described, for example, in the printed document UMTS xx.03, section 1.1, xe2x80x9cCoding for Slotted Modexe2x80x9d.
Alternatively, it is proposed in the printed document ETSI SMG2 UMTS L1Expert Group, Espoo, Helsinki, Dec. 14-18 1998, to use what is known as DTX (discontinuous transmission) time slots (i.e. slots in which no information is transmitted during silent gaps) for the aforementioned interfrequency measurements during the transmission. DTX time slots occur in certain services due to the fact that the transmission capacity needed in each case does not coincide with a possible spreading factor. However, a drawback of this method is that there is an additional delay on the order of the magnitude of the length of the respective DTX time slot (i.e., the duration of the respective slot with active voice).
In another approach, it is proposed, for the compression of the information to be transmitted, to use a smaller spreading factor when using a code division multiple access (CDMA) method as the multiple access method. When a CDMA method is used in a digital mobile radiocommunication system, all subscribers simultaneously have the capability of using the entire available system bandwidth. To avoid collisions between the individual subscribers, the digital data of the individual subscribers are provided with different code sequences which leads to spreading of the respective transmit signal so that the code sequences are also known as spread-spectrum code. The receiving unit of the receiver operates synchronously with the code sequence of the transmit unit of the transmitter and cancels again the spreading from the transmitting end. The despreading process in the receiver only despreads, and thus reduces the bandwidths, of the signal which uses the same and synchronous spread-spectrum code as the receiver. If the transmitter uses a spread-spectrum sequence which leads to lesser spreading of the transmit signal, the information to be transmitted can be compressed in time.
An object of the present invention is, therefore, to provide a further alternative for the compressed transmission of information in a radiocommunication system, particularly a mobile radiocommunication system, which can be implemented in a simple manner.
According to the present invention, the compression is achieved by the fact that the source coding rate used for the source coding in the transmitter is reduced so that the original signal, which is, for example, a voice signal, is transmitted with a lower bit rate. As was described earlier, the compression generates an idle slot in the corresponding transmission frame, which can be used for preparing a handover between different mobile radiocommunication systems.
For the source coding or voice coding, a variable source coding rate is preferably provided so that it is possible to switch between different source coding rates depending on the operating conditions. If, for example, a mobile station acting as transmitter no longer has sufficient reserve capacity for increasing the power for the source coding with the normal (increased) source coding rate, a coding rate can be used for the source coding which is reduced compared with the normal source coding rate. Thus, it is also possible to match the source coding rate to be used for the compression optimally to the available transmission capacity.
An advantage of the present invention consists in that, for voice transmissions, the compression for generating the idle slot only causes a slight impairment of the performance of the mobile radiocommunication system.
The present invention can be used in UMTS (Universal Mobile Telecommunications System) mobile radiocommunication systems to perform handover processes which can proceed, for example, analogously to the GSM (Global System for Mobile Communications) mobile radio standard. Although the AMR (adaptive multirate coder) voice coder with different voice coding rates is already known for the UMTS mobile radio standard, these different voice coding rates are only provided for different transmission conditions. According to the present invention, it is now proposed to use these different voice coding rates also for the frames operated in the slotted mode for generating the aforementioned idle slot by compressing the information to be transmitted, thereby allowing implementation in a simple manner because the frames affected are already known in advance.
Additional features and advantages of the present invention are described in, and will be apparent from, the following detailed Description of the Invention and the Figures.